The present invention relates to load determining devices and has particular application to large industrial scales which support their loads by load indicating transducers.
Industrial scales for weighing large objects are used, for example, to weigh trucks and locomotives and to determine their payload.
A typical industrial scale employs a massive weighing platform or weighbridge. The vehicle to be weighed is driven onto the weighbridge. The weighbridge couples to ground through mounts which include load cells or transducers. These transducers generate electrical signals proportional to the load applied to them. There are several mounts for a weighbridge and the sum of the loads on all the mounts is the weight of the weighbridge, the vehicle it supports, and the payload of the vehicle. Obviously with the weight of the weighbridge and the vehicle known, the payload is the total load less the load of the vehicle and weighbridge.
Each transducer takes the entire load on its mount and undergoes elastic strain when loaded. Strain gauges indicate the strain of the transducer as electrical signals proportional to the load on the transducer. These signals are totalized to indicate load.
The load indicating transducers used in weighbridges have typically been of the cantilever type, tension type, or compression type. The cantilever type has one end rigidly connected to ground and the other end loaded by the weighbridge for bending of the transducer. The strain is measured by the amount of elongation and shortening of the transducer resulting from its bending, and this is related to load on the weighbridge. The tension and compression transducers are loaded along their axes. A tension transducer has balanced forces applied along its axis and away from one another. A compression transducer has balanced forces along its axis and towards each other.
Strain gauges are accurate to reflect elastic strain along a predetermined direction of a transducer. When strain occurs in directions not in the predetermined direction, the load indicated by the strain gauge will not be an accurate indication of true weight.
In a typical application the load is determined by summing up all the loads on all the transducers. If the signals fron these transducers are not directly proportional to the load sensed by them, it will be very difficult to get an accurate signal representing total load. The lack of direct proportionality of a signal with load means that the signal is a non-linear function of load. The relative loading of the transducers with respect to one another varies with the position of the load on the weighbridge and this fact along makes linear-with-load transducers almost necessity.
Horizontal loads on a weighbridge must be accommodated. These loads occur, for example, when a vehicle drives onto the weighbridge. If these loads are allowed to affect the strain in the transducers, then the accuracy of the scale is limited by the amount of error caused by the horizontal loading. While in the example given such loading is transient, it can still be relatively longlasting, for the weighbridge will oscillate. Moreover, wind can cause longlasting horizontal loading. In summary, the load indicating transducers indicate loads accurately only when they are loaded in design directions. The mounts for the load indicating transducers must therefore load the transducers only along the designed directions. This is a difficult task.
U.S. Pat. No. 3,797,593 describes many of these considerations in detail. It also describes a linkage system which isolates a tension loaded transducer from the effects of horizontal forces.
Industrial scales are relatively expensive. A major portion of the expense of an industrial scale is directly related to the height of the scales. The height of a scale measures how much excavation there must be to accommodate the scale in level-with-grade scales and how much supporting framework is necessary in all scales. Expense is directly related to the amount of excavation and framework. Accordingly, it it can be done without sacrificing accuracy, reduced overall height of an industrial scale will mean less cost.